1. Field of the Invention
This invention relates generally to the field of electrical logging and more particularly to the measurement of oxidation-reduction ("redox") potential and hydrogen ion concentration in mud-filled boreholes as an aid in the detection of the presence and contour of subsurface mineral deposits.
2. Description of the Prior Art
In practicing the so-called "redox" logging technique in accordance with the prior art, an inert electrode such as platinum or gold (the "redox" electrode) is immersed in the mud of a shallow wellbore by means of a moving probe. The function of the redox electrode is to "pick up" the potential corresponding to the oxidation-reduction e.m.f.'s existing in the surrounding mud. Measurement is taken with respect to a reference electrode such as a saturated calomel electrode. This potential has been characterized as an absolute and reproducible measure of the response of sediments to spontaneous electron transfer as may take place between an oxidized environment and a reduced environment. Consequently, it is of great importance in mapping of geochemical cells or so-called "roll fronts" associated with uranium and other mineral deposits. Mineral ore occurs along the oxidation-reduction boundary of the cell.
Redox logging has been carried out in accordance with the teachings of several United States patents, for example, U.S. Pat. Nos. 2,691,757 and 3,098,198 to Salimbeni; U.S. Pat. Nos. 3,182,735 to Salimbeni et al; and 3,538,425 to Veneziani. In the last named patent, Veneziani teaches a multi-electrode logging probe inserted in a mud-filled borehole containing at least one redox electrode and one reference electrode individually connected to separate conductors of a probe-running cable.
One problem in such prior art probes lies in the fact that the exposed redox electrodes tend to become clogged by a film of mud as the probe is moved, in which case the Eh measurement is only that of the original mud layer adhering to the tool. This tends to make the speed of redox logging critical. If the speed becomes too great, equilibrium conditions are not reached and again an inaccurate measurement results. A proposed solution has been to maintain a constant logging speed throughout the logging process, but clearly it would be preferable to avoid any limitation of this nature. (See in this connection a published paper entitled The Redox Log, SPWLA, 13th Annual Logging Symposium, May 7-10, 1972.)
In redox logging probes, the prior art preferentially employs a silver-silver chloride metal reference electrode member saturated in an electrolyte such as potassium chloride. In order to establish a fluid bridge, the electrolyte contacts the mud of the borehole through a permeable wick, membrane or other barrier. Pressure compensation means must be provided so that the pressure of the electrolyte increases with the external ambient hydrostatic pressure. If the electrolyte is at least as heavy as the mud, this insures constant electrical contact with the borehole mud through the fluid barrier and prevents contamination of the electrolyte. If, however, under severe mud conditions, a significant positive leak is desired from the electrolyte into the mud at all times, then means must be provided to insure a corresponding positive differential between the electrolyte and the external borehole fluid pressure.
Pressure compensation has been accomplished in a laboratory environment by means of a flexible tubing (see a note entitled "An Improved In Situ pH Sensor for Oceanographic and Limnological Applications" by S. Ben-Yaakov and E. Ruth, Limnology and Oceanography, Vol. 19, #1, January, 1974, pp. 144-151). Exposed, flexible tubing is however physically untenable for well logging applications. The same note also suggests creation of a positive pressure differential by forcing a KCl electrolyte solution into a rubber bulb. The proposed use is in an unconfined oceanographic environment. Such a device would not be mechanically feasible, however, in a borehole. A further problem therefore involved in pressure compensation or creation of a positive pressure differential as outlined above is to devise means of ruggedizing the entire system so that the electrolyte reservoir and the pressure compensation or differential pressure device are both adequately protected from the external environment.